The present invention relates to a pressure sensor, and in particular, to an absolute-pressure type of pressure sensor which has the capability of measuring pressure in absolute values.
Conventionally, a variety of absolute-type of pressure sensors have been known. Of these, each of a Japanese Patent Laid-open (unexamined) Publication No. 1998-90096 (reference publication 1) and a Japanese Patent Laid-open (unexamined) Publication No. 1999-83655 (reference publication 2) discloses an absolute-pressure type of pressure sensor, in which a diffused semiconductor type of pressure sensing chip is liquid-encapsulated by a diaphragm, so that the encapsulating liquid prevents fluid from being contacted to the sensing chip.
Another Japanese Patent Laid-open (unexamined) 1999-295174 (reference publication 3) also discloses an absolute-pressure type of pressure sensor. This sensor employs a metal pressure case that functions as a lid. This case is formed to have a reference pressure space (vacuum space) located on a strain gauge side of a sensor chip. The opposite side of the sensor chip to its strain gauge is brought into contact with the fluid to be detected. Further, a hermetic seal or a feed-through capacitor is mounted to the pressure case so that lead wires are connected to the pressure case. An electrical signal flows out of the sensor chip to the lead wires in the hermetic seal or the feed-through capacitor by way of wires, a board, and lead wires, and then to terminals.
Still, a disclosure of another absolute-pressure type of pressure sensor is provided by U.S. Pat. No. 6,351,996 (reference publication 4). This pressure sensor has a reference sensor space which can be structured relatively easily, without using a metal lid. Practically, the reference pressure space is formed of a pre-mold packaged resin material in such a manner that adhesiveness between a lead frame and the resin material is utilized to seal the pressure reference space.
However, the foregoing various pressure sensors have encountered difficulties as follows. In the case of the absolute-pressure type of pressure sensor disclosed by the reference publications 1 and 2, to avoid air from being mixed with the enclosing liquid, it is required that during manufacturing processes, a space with which the sensor chip is charged be first subjected to vacuum pumping to realize a high vacuum therein, before being subjected to the liquid encapsulation. This way makes the manufacturing processes more complicated, and if the sealing diaphragm is broken, the entire piping will become polluted with the enclosing liquid.
The absolute-pressure type of pressure sensor disclosed by the reference publication 3 is structured complicatedly and needs a lot of electrical connections. In addition, further electrical connections should be placed within the pressure case that is formed with welding, which results in complicated assembling work of the sensor. Reliability will be reduced, which attributes to the fact that there are lots of electrical connections.
Further, in the case of the absolute-pressure type of pressure sensor disclosed by the reference publication 4, reliability of the sealing performance is low, due to the fact that the adhesiveness between the lead frame and the resin material gives the sealing of the pressure reference space.
Considering the above difficulties, as shown in FIGS. 9 to 11, the inventors of the present application provided an absolute-pressure type of pressure sensor, which is able to overcome the foregoing difficulties. To be specific, a strain gauge 9 is formed on one surface of a diaphragm 8 composing part of a pressure detecting device 1, around which a hermetic seal terminal 2 is fixedly coupled by welding. Wires 3 are used to establish electrical connections between an input/output terminal 7 of the hermetic seal terminal 2 and the strain gauge 9. After those connections, a metal-made lid 4 is disposed over the pressure detecting device 1 and welded to the hermetic seal terminal 2 in vacuo. A fluid receiving opening 6 is formed in the body of the device 1. This pressure sensor provides a reference pressure space 5 with a higher reliability, which is formed between the lid 4 and the pressure detecting device 1.
However, there is the drawback that mounting the pressure sensor is made difficult, because fluid receiving opening 6 agrees with the input/output terminal 7 in terms of their directions, along which the hole 6 is formed as well as the input/output terminal 7 is attached.